1. Field of the Invention
The present invention relates to an EL (electro luminescent) element drive circuit and an organic EL display device using the same drive circuit and, in particular, the present invention relates to an organic EL display device suitable for high luminance color display, with which white balance on a display screen of a display device of an electronic device such as a portable telephone set or a PHS, etc., can be easily regulated or variation of luminance can be reduced, by luminance regulation of R (red), G (green) and B (blue) colors.
2. Description of the Prior Art
An organic EL display panel of an organic EL display device, which is mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance) and includes 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines, has been proposed and the number of column lines and the number of row lines of such organic EL display panel tend to be further increased.
An output stage of a current drive circuit of such organic EL display panel includes an output circuit constructed with, for example, current-mirror circuits, which are provided correspondingly to respective terminal pins of the panel, regardless of the type of drive current, the passive matrix type or the active matrix type.
One of the problems of a conventional organic EL display device is that, when the voltage drive is used to drive terminal pins thereof as in a liquid crystal display device, a display control becomes difficult and luminance variation becomes conspicuous due to difference in luminous sensitivity between R, G and B display colors. For this reason, the organic EL display device has to be current-driven. However, even when the current drive is employed, ratio of light emission efficiency for drive currents of R, G and B colors is, for example, R:G:B=6:11:10, which depends upon luminescent materials of the organic EL elements.
In view of this, it is necessary, in a current-drive circuit of an organic EL color display device, that white balance is obtained on a display screen thereof by regulating luminance of each of R, G and B colors correspondingly to luminescent materials of EL elements for respective R, G and B colors. In order to realize such white balance regulation, regulation circuits for regulating luminance of respective R, G and B colors on the display screen are provided.
Incidentally, JPH9-232074A discloses a drive circuit for organic EL elements, in which the organic EL elements arranged in a matrix are current-driven and a terminal voltage of each organic EL element is reset by grounding an anode and a cathode of the organic EL element. Further, JP2001-143867A discloses a technique with which power consumption of an organic EL display device is reduced by current-driving organic EL elements with using DC-DC converters.
It is usual in the conventional organic EL display device that the current-drive circuit of the organic EL display device generates drive currents for driving organic EL elements connected to respective column line pins by current-amplifying reference currents for R, G and B display colors and the regulation of drive currents for obtaining white balance is performed by regulating the reference currents for the respective R, G and B display colors.
In order to regulate the reference currents for the respective R, G and B colors, each of reference current generator circuits of a conventional drive current regulator circuit includes a D/A converter circuit of, for example, 4 bits and the reference currents for the respective R, G and B display colors are regulated by setting a predetermined bit data for each of R, G and B display colors at 5 μA intervals within a range, for example, from 30 μA to 75 μA. With the fact that various organic EL materials have been developed recently, the luminance regulation for realizing white balance, which is realizable by the D/A converter circuits, is not enough since the dynamic range of regulation is as rough as 4 bits.
However, if the number of bits of the D/A converter circuit for luminance regulation of each of R, G and B display colors is increased, the terminal pins of the column lines have to be driven by a plurality of drive ICs and each drive IC has to drive a plurality of terminal pins. As a result, the current output characteristics of drive circuits of current sources corresponding to the respective terminal pins is varied and so luminance variation of the organic EL display panel driven thereby becomes conspicuous.